The present invention relates to an apparatus and method for determining aircraft velocity and position, and more particularly, to an apparatus and method for determining the longitudinal and lateral ground velocity of an aircraft and for providing positional data for navigation of the aircraft.
Knowledge of an aircraft's velocity with respect to the ground provides important data that is useful for safe navigation. For example, during aircraft landings actual ground speed data is essential for the proper operation of the wheel brake antiskid control system, and more specifically the hydroplaning and touchdown spin-up logic thereof. An aircraft's ground speed can also be compared to a conventional barometric air speed reference to detect wind shear conditions.
Information of an aircraft's position with respect to the ground can also be useful for navigation. Such data can be used to track the location and path of the aircraft over the ground. Deviations between the actual and planned flight paths indicate whether the aircraft is off course. Ground position data can also be used to stabilize an aircraft. For example, when an aircraft rolls side-to-side about the longitudinal axis defined by its fuselage, it creates a sinusoidal flight path with respect to the ground. The frequency and amplitude of this sinusoidal path are proportional to the roll frequency and roll angle of the aircraft. The roll frequency and roll angle may be determined from ground position data corresponding to the sinusoidal flight path, thereby allowing the pilot to take corrective measures.
One known means for determining velocity of an aircraft is shown in U.S. Pat. No. 4,162,509 (Robertson). This reference discloses a pair of photodiode array cameras which are mounted a fixed distance apart on a vehicle. The first camera makes a first image of the earth's surface, and at some interval later the second camera makes a second image of the earth's surface. A correlator makes an element-by-element comparison of the two images to see if they correspond to the same area of the earth's surface. If not, the time interval is adjusted, and new images are made and correlated. The velocity of the aircraft is determined by dividing the known distance between the cameras by the time interval which produces complete correlation.
While the Robertson system provides a measure of ground velocity, it also has several disadvantages. First, Robertson continually samples and correlates data until complete correlation is achieved. The more data sampled and correlated the slower the update time for computing velocity. This process is also prone to error because it repeatedly disregards existing data and samples new data. Second, Robertson will have difficulty achieving complete correlation when the vehicle is rapidly accelerating or decelerating because the time interval necessary to achieve complete correlation will be changing. Third, Robertson only teaches how to determine the velocity of the aircraft in one dimension at high velocity. This reference suggests that velocity can be determined in two dimensions at low velocities, but it is not clear how this is done.
Another shortcoming of the Robertson system is that it is subject to motion blurring because its cameras consist of photodiode array image sensors. When the aircraft is moving, incident light will be spatially blurred over a plurality of photodiode elements. In effect, motion blurring is due to the integration of a scene translated photodiode-by-photodiode over the distance corresponding to the aircraft velocity multiplied by the exposure time. Thus, a point source of light will be recorded as a bar over a plurality of photodiodes, wherein each photodiode has an exposure level which is only a fraction of the level to be expected by a record of a stationary scene. This blurring acts as a low pass filter which reduces the feature definition and limits the sharpness of a correlation calculation. This phenomenon will be particularly noticeable at low levels of radiation requiring a long exposure time.
U.S. Pat. No. 3,545,268 (Von Struve) discloses a satellite ground track alignment sensor. According to the disclosure, a satellite is provided with two front sensors and two back sensors having divergent axes that all project towards the earth's surface. The sensors detect characteristic signatures of the earth's surface, such as an infrared signature. The output of a first front sensor is delayed by the time it takes its corresponding back sensor to view the same area of the earth's surface. The outputs of these two sensors are correlated. If the satellite is properly oriented with respect to its projected surface ground track, the outputs of the front and back sensors should exactly match. If however, there is a heading error, the correlation calculation of the front and back sensors will yield the magnitude of the heading error. The output of each front sensor is also correlated with the output of its diagonally disposed back sensor. These two resulting correlation calculations are compared to determine the polarity of the heading error with respect to the ground track.
The Von Struve system is limited to determining the magnitude and polarity of misalignment from a projected surface ground track. The reference does not teach how to generate ground position data that could be useful for making other navigational determinations (e.g. roll frequency). Moreover, the Von Struve system is also subject to some of the shortcomings of the Robertson system. For example, Von Struve must provide a means to adjust the time delay when the satellite is accelerating or decelerating.